2 cos^3 x - cos^2 x + 2 cos x - 1 = 0

По моему так

2 cos^2 x - 3 cos x +1 = 0сosx=y2y²-3y+1=0D=(-3)²-4*1*2 = 9-8=1y1=(3+1)/4=1y2=(3-1)/4=1/2=0.5 cosx = 1x=2pi n,  n є Z  cosx=1/2x=±pi/3 + 2pi n,   n єZ